The present invention relates to a method and apparatus for treating recycled fibers and/or fraction exiting from the vortex cleaning as mill reject. As is known, sources of recycled fibers include both the so called broke of a paper machine, which may be rerecycled as raw material, and the actual post-consumer waste paper and board. The present invention especially relates to the treatment of such recycled fiber materials so that the fillers therein may be recovered as well as possible and returned back to the process as efficiently and economically as possible.
In the short circulation of paper machines manufacturing filler-containing paper grades and especially coated paper grades a lot of mineral and pigment fraction is discharged nowadays from the process as mill reject of a vortex cleaning plant, which of its content might be utilized as raw material for paper, but is of its particle size too coarse.
In the short circulation of paper machines manufacturing SC grades and grades containing other fillers, the mineral fraction exiting the vortex cleaning as mill reject is the coarse portion of the mineral fraction added in the pulp dosing, the particle size being generally more than 10 xcexcm.
In the short circulation of paper machines manufacturing coated paper grades the mineral fraction exiting the process as mill reject mainly contains non-dispersable coating layer of coated waste paper. The coating pigment has not dispersed in the dispersion system to particles fine enough. The fraction of the coating pigment, which has not been dispersed and is of its size generally more than 10 xcexcm, is rejected in the vortex cleaning of the short circulation.
The same applies to systems for recycled pulp, in which coated newspaper or like heavily coated raw material for recycled paper is used as raw material. In the defiberizing system for recycled pulp the coating pigment of the coated paper is released from the actual fiber layer of the paper more or less in sheet-like fractions, which are partially dispersed due to the process. The dispersion is, however, not complete and that is why these coating pigment particles which have remained non-dispersed are discharged from the screening stage of the process as reject of the vortex cleaning. However, the majority of the particles would be of their raw material content filler material usable in the paper manufacture, which may possibly be added to the fiber suspension in the later stages of the paper manufacture. The size of these particles is such that said particles cause problems in the paper manufacture if they are not dispersed and thus made qualified to be used as fillers.
U.S. Pat. No. 4,504,016 discloses a process and apparatus for cleaning chemical cellulose by screening. The problem underlying the patent is to increase the yield of chemical pulp in the production thereof. In the process the cellulose is sorted and a coarse fraction, i.e. knots and shives, is treated in a refiber and further sorted. Since it is a question of a chemical pulp manufacture process there are no further substances e.g. filler material involved in the process. Also, the document teaches the removal of the rejects of the hydrocyclone out of the system.
U.S. Pat. No. 4,167,438 discloses a process and apparatus for preparing and cleaning fibrous material, e.g. mixed waste paper. The publication teaches the combined recycle of both fibers and fillers back to use. There is no suggestion that the process could be used for separating and treating the mere filler fraction of the waste paper pulp.
In an arrangement in accordance with the present invention the loss in fillers/minerals exiting the vortex cleaning as mill reject is diminished by treating fraction having a concentrated mineral content in the vortex cleaning plant for dispersing mineral fractions and returning them back to the process.
Advantages obtained by utilizing the method and apparatus in accordance with the present invention are, for example, following:
Filler/mineral, water, chemical, heat and fiber losses are as low as possible. Only useless fraction and fraction that cannot be changed into a form that can be re-used are discharged in a very concentrated form.
Dispersion of mineral particles is based on internal shear forces of the suspension, in other words mechanical wear is minimal.
Investment costs of the system are very low. Technical realization is carried out with commercial apparatuses, repayment period is short.
System is easy to build, i.e. it may be mounted to the already existing systems by adding a screening apparatus fractionating the mineral fraction and wood fraction separate and a new kind of cleaner for the discharge of reject in a very concentrated form after the last cleaner step and by adding a treatment stage dispersing the mineral fractions thereafter.
Treatment is carried out, for example, in the short circulation of a paper machine for each machine individually, whereby, for example, the water circulations between the machines are not mixed.
System is a continuously operating part of a short circulation of a paper machine, or of a screening system. In other words, the process conditions are constant and the operation trouble-free. Process adjusts itself, for example, if the amount of the coarse fraction increases, the system returns only the dispersed fraction to the process and the rest is discharged from the system.